Ringer isolator circuits are used to eliminate electrical noise that results from ringer ground connections during the talking state of a telephone call while the called and calling parties are connected to each other. This is accomplished by keeping the telephone ringer isolated or disconnected from ground except for the periods when a central office ringing signal voltage is applied to ring the telephone.
ANI mark circuits (also referred to as station identification circuits) are used with plural party lines to identify toll calls dialed by a selected party (customarily the tip party as distinguished from the ring party) for billing purposes in an automatic number identication system.
Prior to this invention it has generally been the practice to wire ringer isolator and ANI mark circuits directly into the telephone sets. It also has been a custom to incorporate the ANI mark circuits into the telephone sets at the time of manufacture, but to make the circuit connections in such a way that the ANI mark circuit remains inactive or disabled until such time that it is desired to install the telephone at a tip party's station on a two-party line.
Because of this custom, service calls are frequently needed when a telephone set is later assigned to a new or different subscriber for the purpose of converting the set in some cases to single party or ring party use and in other cases to tip party use.
In many cases existing telephones already on subscriber's premises are not equipped with ANI mark circuits. If they require conversion to tip party use, service calls are required, and access to the telephone is needed where it is desired to use the type of ANI mark circuit that must be physically incorporated or wired into the telephone itself. Likewise, house calls are required to equip subscribers' telephones with the type of ringer isolator that is assembled in the telephone itself.
Many times, however, no one is at home at the subscriber's home to admit the service man when he arrives. Return calls are therefore needed, and special arrangements sometimes have to be made to enable the service man to gain access to the telephone in the subscriber's home. This leads to increased costs in providing service to the subscriber. For example, each trip made by a telephone company's service man to a subscriber's home presently costs forty to fifty dollars even for occasions where he finds no one at home.
To avoid these problems and to lower the costs attendent with service calls it has been proposed to design the ANI mark equipment in such a way that it need not be physically housed in the telephone set. One example of such an ANI mark circuit is shown in FIG. 2 of the U.S. Pat. No. 4,001,512 which issued to D. F. Proctor on Jan. 4, 1977. Another example of this type of circuit is described in U.S. Pat. No. 4,054,942 which issued to C. W. Chambers, Jr. on Oct. 18, 1977.
Both of these prior ANI mark circuits may be installed without requiring access to the subscriber's telephone. As a result, no changes are required in the telephone itself or the wiring to the telephone to convert the telephone station to tip party use on a two-party line. The ANI mark circuits described in the above-mentioned patents, however, have some serious drawbacks.
In both patents, for example, the circuit designs are such that insertion or line losses are created by electrical components which are connected in the subscriber loop itself in series with one or both of the tip and ring conductors. In U.S. Pat. No. 4,054,942 and in one of the embodiments in U.S. Pat. No. 4,001,512 there is the additional problem of electrical noise which is created by grounding one of the two subscriber line conductors (usually the tip conductor) through a resistor, but not the other conductor in the course of establishing the ground mark. The circuits described in U.S. Pat. No. 4,001,512 also create a longitudinal line imbalance problem because certain components require connection in series with one side of the subscriber loop or line and not the other.
In another type of prior ANI mark circuit, the base-emitter junction of a bipolar transistor is connected across a resistor that is in series with the tip conductor of the tip party's drop, and the collector of the transistor is connected to ground through a resistor (usually 2.6 K.OMEGA.). With this circuit design, loop current flow in the tip party's loop circuit turns on the transistor to create a difference in the currents flowing in the tip and ring conductors. The resulting current difference is sensed by ANI equipment in the central office. This ANI station identification circuit is referred to as the differential type and cannot be employed with the longitudinal type of identification in which a current path to ground is sensed at the central office to identify the calling party as the tip party.
Examples of prior ringer isolator circuits are described in U.S. Pat. No. 3,916,111 issued to H. W. Ott on Oct. 28, 1975, U.S. Pat. No. 3,303,289 issued to M. S. Hawley et al on Feb. 7, 1967, and U.S. Pat. No. 3,230,316 issued to P. M. Hunt on Jan. 18, 1966.
These patented ringer isolated circuits for the most part are normally wired into the telephone set itself and therefore present the problem of obtaining access to the telephone to make the installation.
Another type of ringer isolator unit marketed by the Cook Electric Company, however, is especially adapted to be connected outdoors to the subscriber's drop to avoid the problem of requiring access to the telephone itself. The Cook Electric ring isolator is a.c. coupled by a first capacitor to the subscriber's line to couple the ringing signal to a diode rectifier for rectification. The recitified ringing signal current charges a second capacitor which is shunted across a neon bulb. When the voltage built up across the second capacitor reaches the firing potential of the neon bulb, the bulb conducts to emit light which is directed to control the resistance of a light sensitive resistor. The light sensitive resistor controls current to the gate of a SCR (silicon control rectifier) which is connected in a bridge between the telephone's ringer and ground. Impingement of the light from the neon bulb on the light sensitive resistor turns the SCR on to connect the telephone ringer to ground for ringing the telephone. Although this ring isolator operates satisfactorily, it has a number of drawbacks.
First, the Cook ringer isolator unit is large and bulky. Second, the two capacitors in the unit described above are not protected against voltage faults or transients. Third, it operates only in response to an a.c. input and is not equipped to handle superimposed ringing. Fourth, it is not equipped to optionally handle elevated voltages.
The present invention avoids the foregoing problems and drawbacks as well as offering additional advantages as will become apparent from the following summary and description.